Apparatus for lithologic logging of underground formations by acoustic vibrations



F. H. REDWINE ETAL 3,486,375

APPARATUS FOR LITHOLOGIC LOGGING OF UNDERGROUND FORMATIONS BY ACOUSTICVIBRATTONS Filed Jan. 22, 1968 2 Sheets-Sheet l E m N mgm 5 m wfi m WRAM4, NE w A Z a a v/M 30,1959 F. H. REDWINE ETAL 3,48 7

APPARATUS FOR LITHOLOGIC LOGGING 0F UNDERGROUND FORMATIONS BY ACOUSTICVIBRATIONS Filed Jan. 22. 1968 ,2 sheets-sh t 2 Tamil.

TLETLL.

ATTOR s United States Patent Ofiice 3,486,375 Patented Dec. 30, 19693,486,375 APPARATUS FOR LITHOLOGIC LOGGING OF UNDERGROUND FORMATIONS BYACOUSTIC VIBRATIONS Fletcher H. Redwine, Irving, James L. Newman,Richardson, and William F. Osborn, Dallas, Tex., assignors to CoreLaboratories, Inc., Dallas, Tex.

Filed Jan. 22, 1968, Ser. No. 699,626 Int. Cl. E21b 49/00; G01n 9/18 US.Cl. 73152 14 Claims ABSTRACT OF THE DISCLOSURE Lithology loggingapparatus including a transducer bonded to the drilling rig to generateelectrical signals in accordance with the vibrations of the drillstring, with means to modify the signals by attenuating at least thefrequency components below about kHz., rectifying them, and integratingthem at a time constant on the order of 2.5 seconds, and means forvisually presenting the modified signals.

The present invention relates to logging apparatus for use in analysisof the lithology of the underground formations being penetrated by thedrill bit of a rotary drilling rig.

It is well known in the petroleum industry that the driller andgeologist are constantly seeking new and more reliable means forascertaining during the drilling of a well the nature of the stratatraversed by the borehole so that they will know when geologic horizonspossibly productive of oil or gas are being reached, and the drillingmay be interrupted and tests conducted to determine whether productionis in fact feasible. It has accordingly been a common practice tomaintain a lithologic log of the drill cuttings. However, cuttings logsare subject to the recognized disadvantage that it is diflicult, if notimpossible, to determine with certainty the depths from which therespective samples of cuttings originated, due for example, to themixing of the cuttings in the turbulent stream of drilling mud and theinaccuracy in calculating the lag time required for the mud to travelfrom the bottom of the borehole up the irregular annular space betweenthe drill string and the sidewall of the borehole to the top of thewell, and in estimating the slippage of the cuttings relative to the mudstream. Moreover, in deep wells it may take as long as an hour or morefor the cuttings to travel from the bottom to the surface, so that thedrilling may have progressed a substantial depth before analysis ispossible.

Other well-known methods of determining the nature of the formationstraversed by the borehole, such as electric logs, radiation logs, soniclogs, sidewall samples, and so on, all involve the interruption of thedrilling operation, and most also involve a round trip" of the drillstringthat is, removing the drill string from and subsequently replacingit in the borehole, section by section, so that an analytical sonde maybe lowered into the borehole on a wire line. This, of course, meanssubstantial and expensive down time for the drilling rig. It alsointroduces a possible element of error due to lack of proper correlationbetween the depths as measured by the driller and those indicated by thewireline device.

It has previously been hypothesized that various rock formationsgenerate characteristic acoustic vibrations upon being broken by a drillbit and that, by detecting and analyzing such vibrations, the nature ofthe formations may be ascertained even as they are being drilled.However, heretofore this hypothesis has not advanced beyond thetheoretical or speculative stage and no practical and reliable systemhas yet been proposed for detecting the vibrations at the surface andpresenting them in such manner as to permit determination of thelithology of the formations penetrated.

We have now developed practical apparatus for detecting the acousticvibrations produced by drilling underground formations, for convertingsuch vibrations into fluctuating signals and for so modifying suchsignals and recording them in a form susceptible of ready and reliableanalysis to determine the lithology of the source formations. Thus, wehave confirmed the aforementioned hypothesis and have made available tothe driller and geologist for the first time a practical tool permittingsubstantially instantaneous determination of lithology during theprogress of drilling.

In the drawings:

FIGURE 1 is a somewhat diagrammatic illustration of a typical rotarydrilling rig, showing a detecting device forming an element of anillustrative embodiment of the invention fastened to the swivel whichrotatably supports the drill string.

FIGURE 2 is a schematic diagram of an illustrative electrical circuitwhich converts the signals produced by the detecting device tointeligible form and records them.

FIGURE 3 illustrates a typical log as produced by the apparatus ofFIGURES 1 and 2.

The typical drilling rig diagrammatically illustrated in FIGURE 1includes a derrick 10 with a. draw works (not shown) supporting atravelling block 12 with a depending hook 14 engaging the bail 16 of a.swivel 18 which rotatably supports the drill string. The top joint ofthe drill string is a square-section kelly 20 which is driven by therotary table 22.

Fastened to the outer face of the swivel 18 is a transducer 24 whichconverts the mechanical vibrations of the drill string and swivel intoelectrical signals which are conducted to the associated instrumentationin a mobile laboratory near the drilling rig, through an electricalcable 26, for example a coaxial cable, which may be taped along therotary hose 28 and the stand pipe 30 so as to be out of the way ofoperations around the rig.

The transducer 24 may, for example, be a piezoelectric crystallinematerial, such as a ceramically fused and ground wafer of bariumtitanate or lead zirconium titanate, or a quartz or tourmaline crystal,having a resonance frequency between and 500 kHz., for example, 183 kHz.The crystal may be bonded directly to the swivel 18, for example bymeans of an epoxy adhesive.

However, in FIGURE 2, a preferred detachable mounting arrangement isshown, in which a piezoelectric crystal 32 is mounted in a recessedsupport member 34 of insulating material, such as a thermosetting resin,which is removably fastened to a mounting plate 36 welded or otherwisebonded to the swivel 18, by means of screws 38 extending through thecircumferential flange 34a of the support member 34 and threaded intothe mounting plate 36. The inner face of the crystal 32 abuts the outersurface of a fixed electrode 40 formed integrally with or brazed to themounting plate 36 while its outer face is engaged by a movable electrode42 mounted in the recess of the mounting plate 36 and urged outwardlythereof by a coil compression spring connected at its inner end to themovable electrode and at its outer end to a terminal screw 42 extendingthrough the outer wall of the support member. A gasket 44 of resilientmaterial, such as rubber or neoprene, is compressed between the flange34a and the mounting plate 36 to seal the crystal from moisture anddirt. The inner conductor 26a of the coaxial cable 26 is connected tothe terminal screw 42. The shield braid 2611 may, if desired, be securedto a terminal screw threaded into the mounting plate 38 and thus beconductively connected to the fixed electrode 40, or it may be leftungrounded at this end to obviate the possibility of difficulty due to aground loop.

As will be understood, the mechanical vibrations of the swivel 18resulting from the drilling operation are transmitted to the crystal 32through the fixed electrode 40, causing the crystal to generate anelectrical voltage varying in accordance to such vibrations, and thiselectrical voltage is picked up by the electrodes 40 and 42 and conveyedthrough the cable 26 to the electrical modifying and recordingapparatus. As shown in FIGURE 2, this apparatus includes an amplifier 46which increases the amplitude of the electrical voltage to a usablevalue. The amplified signal is passed through a filter 48 which limitsthe range of frequencies therein.

We have found that in order to reduce the background electrical noiseproduced by local vibrations of the drilling rig due, for example, tooperation of the rotary drive motor, drive train and rotary table, themud pumps, etc., as Well as to vibrations of the swivel and drill stringproduced by friction of the swivel bearings, by the pulsating flow ofdrilling mud, by rubbing of the pipe against the sidewalls of theborehole, and so on, the lower frequency components of the signal, forexample, all those below a frequency on the order of to 30 kHz., shouldbe attenuated. Preferably, but not necessarily, the filter may alsoattenuate higher frequency components above an upper cutoff frequencydependent upon the type of bit being used. Where a conventional tri-conerock bit is used, it has been found that a pass band of about 30 to 40kHz. may be used effectively. Where a button biti.e., a multi-cone bithaving buttons of a hard material such as tungsten carbide welded to theroller cones in place of the usual integral projecting teeth-is used, ahigher cutoff frequency to provide a pass band of about 30 to 130 kHz.has been found more advantageous, although the lower cutoff frequencymay be reduced as low as 10 kHz. without intolerable background effects.

The modified signals appearing at the output of the filter 48 are fedfirst to a rectifier 50, for example, a silicon diode, which convertsthe high frequency alternating current to pulsating direct current, andthen to an integrator circuit 52, having a time constant between 0.25and 100 seconds, for example a conventional R-C integrator circuitincorporating a 2,500-ohm resistor and a 100- microfarad condenser, togive a time constant on the order of 2.5 seconds, which further modifiesthe signal by averaging out the individual high-frequency pulsations togive a smoothed low-frequency signal susceptible of being transducedinto an intelligible visual form, for example by recording on a movingstrip chart.

The further modified signal is fed to a visual indicator device, such asan oscillograph, or preferably, as shown in FIGURE 2, a conventionalmillivolt strip chart recorder 54 whereby the signal is recorded as theordinate of a graph whose abscissa may be either time or depth.Preferably, the chart drive of the recorder is coupled through a wireline device to the swivel 18, and the chart calibrated and synchronizedso that its horizontal divisions represent the depth of the borehole infeet.

FIGURE 3 illustrates a typical chart produced in the manner indicated.As may be seen, the abscissa of this chart represents the depth of theborehole, with the horizontal divisions marked to indicate the depth infeet. The ordinate represents the amplitude of the modified electricalsignal. The graph illustrated is typical of the type we have observedduring drilling.

The lower amplitude signals indicated at 56 are typical of thoseproduced when drilling through softer formations such as shale oranhydritic shale having a certain degree of plasticity. The higheramplitude signals indicated at 58 are typical of those produced byharder formauons such as limestone or lithified sandstone which are moreperfectly elastic in their mechanical properties. The high amplitude,high frequency signal 60 is indicative of the type experienced indrilling through an extremely hard formation, for example, one rich inminerals such as pyrite.

During the drilling of wells in which charts of the type illustratedwere being recorded by the equipment of our invention, neither we northe experienced drillers involved have been able to detect on thedrilling floor any differences in the operation of the drilling rigwhich could possibly have been interpreted to indicate significantdifferences in the nature of the formations being drilled, such as arerepresented by the readily apparent changes in the signals recorded onthe chart.

It will be readily appreciated that a skilled geologist, afterfamiliarization with the type of signals produced by the presentequipment, would be readily able to correlate such signals with downholelogs made in other wells in the area, including even diverse types oflogs, such as electric logs, thereby ascertaining quickly during thecourse of drilling the exact point in the geologic profile which hasbeen reached, and when formations possibly containing commerciallyproducible accumulations of oil or gas are expected to be penetrated.

It will therefore be appreciated by those familiar with petroleumexploration techniques, that the present invention provides a practicalapparatus for accomplishing the aforementioned and other desirableobjectives. However, it should be emphasized that the particularapparatus described herein and shown in the accompanying drawings isintended as merely illustrative of the principles of the invention andnot as restrictive of the scope thereof, which is defined only by theappended claims.

We claim:

1. Apparatus for logging the lithology of underground formations beingtraversed by the drill bit of a rotary drilling rig comprising atransducer for converting mechanical vibrations to electrical signals,said transducer being fastened to a non-rotating part of the drillingrig above ground, an amplifier for amplifying the electrical signalsproduced by said transducer, a filter for modifying said signals byattenuating at least the frequency components of said signals below apredetermined cutoff frequency, a rectifier and an integrator circuitthrough which said signals are transmitted to convert said signals topulsating direct current and further modify the same in accordance withthe time constant of said integrator circuit, and an indicator means forpresenting such further modified signals in visible form.

2. Apparatus as claimed in claim 1 in which said indicator means is astrip chart recorder.

3. Apparatus as claimed in claim 2 in which said recorder includes achart drive coupled to said drilling rig to plot said signal as afunction of the depth of said drill bit.

4. Appartaus as claimed in claim 1 in which said transducer is apiezoelectric crystalline material.

5. Apparatus as claimed in claim 4 in which said transducer is a fusedtitanate material.

6. Apparatus as claimed in claim 1 in which said transducer has a usableoutput between and 500 kHz.

7. Apparatus as claimed in claim 1 in which said transducer is bonded tosaid part by means of an adhesive.

8. Apparatus as claimed in claim 1 in which said transducer is carriedby a support assembly which is removably attached to a mounting platebonded to said part of the drilling rig.

9. Apparatus as claimed in claim 1 in which said transducer is fastenedto the swivel which rotatably supports the drilling string.

10. Apparatus as claimed in claim 1 in which said filter is a high passfilter with a cutoff frequency between about 10 and about 30 kHz.

11. Apparatus as claimed in claim 1 in which said drill bit is of thehardened button type and said filter is a band pass filter with a passband on the order of 10 to 130 kHz.

12. Apparatus as claimed in claim 1 in which said drill bit is of thetoothed multi-cone type and said filter is a band pass filter with apass band on the order of 30to 40 kHz.

13. Apparatus as claimed in claim 1 in which said integrator circuit hasa time constant between .25 and 100 seconds.

14. Apparatus for logging the lithology of underground formations beingtraversed by the drill bit of a rotary drilling rig comprising apiezoelectric transducer for converting mechanical vibrations toelectrical signals, said transducer being attached to a non-rotatingpart of the drilling rig which functions in the support of the drillstring; an amplifier for amplifying the electrical signals produced bysaid transducer; a filter for modifying said signal by attenuating allfrequencies below a frequency on the order of 30 kHz.; a rectifier andan integrator cir- References Cited UNITED STATES PATENTS 2,161,2566/1939 Karcher 73-152 X 2,469,383 5/ 1949 Gibbs et a1.

2,550,420 4/ 1951 McNatt 73151.5 2,769,867 11/ 1956 Crownover et al.

2,810,546 10/1957 Eaton et'al.

JERRY W. MYRACLE, Primary Examiner US. Cl. X.R. 7371.4

